FIELD OF THE INVENTION
The present invention relates to elements of an apparatus which signals intrusion into an enclosed area.
Apparatuses of this type are known which consist substantially of a pair of yielding tubes filled with fluid, positioned side-by-side and embedded underground, extending for the entire perimeter of the surface to be covered, such pipes are also provided at one of their extremities with an equal number of receiver transducers; in this way, when the fluid contained in one of the two pipes receives an impulse from the outside, due for example to the passing of a person over the terrain, differentiated signals are sent to the transducers of a kind such as to activate an alarm.
This principle is of great value, inasmuch as it means that even very weak signals can be captured, such as for example those caused by persons crawling on the terrain. Additionally, in this manner there is actuated a wholly hidden signaller which it is impossible to deactivate except from the inside of the protected area.
Nevertheless, an apparatus of this type has a considerable defect; for, as will be appreciated, the alarm signal has to be activated by very small impulse variations inasmuch as the pressure brought to bear by a person on the terrain generates to the fluid of one of the two pipes a very slight impulse as compared with the rest condition.
The alarm, therefore, must function when the signals coming from the two transducers are hardly at all differentiated--a differentiation of the order of a few millivolts. The result of this is that environmental factors such as wind, rain or very busy roads and nearby railways can frequently cause small variations in the impulses to the fluids of the two pipes, of a kind that can activate the alarm at any time.
The dependability of this device thus falls down precisely because the two transducers in this case would be supplying signals of the order of a few volts, so that is would be very easy for the signal of one pipe to deviate from the signal of its adjacent pipe by the few millivolts necessary to activate the alarm, which, as has been seen, must commence to function when a minimum difference exists between the two signals.
This occurs inasmuch as the impulse from the fluid contained in the pipes to the alarm arrives through two sensors each connected to a membrane positioned at the end of each tube, in direct contact with the fluid.
The behaviour of the membranes is not always definite, and in any case not linear, thus making it impossible to perceive useful signal levels beyond a certain limit by appropriately countering any external disturbances due, as has been seen, to atmospheric and environmental factors.